This invention relates to a method of purifying N-(2-hydroxyethyl)-2-pyrrolidone. N-vinyl-2-pyrrolidone, which is derived through intramolecular dehydration reaction of N-(2-hydroxyethyl)-2-pyrrolidone, is useful as starting monomer for poly-N-vinyl-2-pyrrolidone which has wide variety of utilities as a starting material for pharmaceuticals, food additives and cosmetics.
As a method of preparing N-(2-hydroxyethyl)-2-pyrrolidone, one comprising ring-opening and adding y-butyrolactone to 2-aminoethanol in liquid phase to form 4-hydroxy-N-(2-hydroxyethyl)-butanamide intermediate product, and then heating said intermediate product to cause its intramolecular dehydration reaction, while using water as the catalyst [Official Patent Gazettes of JP1972-21420 B, JP1974-20585 B and JP1979-22973 B (=U.S. Pat. No. 3,867,405)] is known.
Reaction products of the above method have complex composition composed of N-(2-hydroxyethyl)-2-pyrrolidone, water formed of the dehydration reaction, compounds having lower boiling points than N-(2-hydroxyethyl)-2-pyrrolidone (hereafter referred to as low-boiling component) and compounds having higher boiling points than N-(2-hydroxyethyl)-2-pyrrolidone (hereafter referred to as high-boiling component). Hence, for obtaining high purity (at the lowest 99.9% by weight) N-(2-hydroxyethyl)-2-pyrrolidone from the reaction products by removing the high-boiling component and low-boiling component, purification steps such as distillation is necessary. However, none of above cited Official Patent Gazettes contained any disclosure on a method for recovering high purity N-(2-hydroxyethyl)-2-pyrrolidone from said reaction products.
Whereas, as for N-methyl-2-pyrrolidone which is one of pyrrolidones but has entirely different physical properties from those of N-(2-hydroxyethyl)-2-pyrrolidone, methods of purification and recovery thereof have been disclosed in many Official Gazettes.
JP2,785,629 and JP3,024,414 disclose a production process of pyrrolidones characterized by distilling the low-boiling component off from the top of a first distillation column.
JP Kokai (laid-open) Gazette 2001-2638 A discloses a production process of high purity pyrrolidones, which comprises distilling the low-boiling component including unreacted amine and water off from the top of a first distillation column, setting the bottom liquid temperature of the first distillation column at 90-200xc2x0 C. and the average residence time of the bottom liquid, for 10 minutes to 8 hours; while extracting from the bottom of the same column the bottom liquid of an amount corresponding to 2-15% by weight of the pyrrolidones contained in the feed; and recovering the remaining part of the pyrrolidones as side cut. JP Kokai Gazette 2001-2640 A also discloses a production process of high purity pyrrolidones which comprises setting the bottom liquid temperature in the first distillation column at 90-200xc2x0 C. and the average residence time of the bottom liquid, for 10 minutes to 8 hours, while removing from the bottom of the first distillation column the bottom liquid of an amount corresponding to 2-15% by weight of the pyrrolidones contained in the feed, distilling the low-boiling component and the remaining pyrrolidones off from the column top, then supplying said column top distillate to a second distillation column, and distilling the low-boiling component off from the column top and recovering the pyrrolidones from the bottom of said column.
Thus, there is no prior art which disclosed a method for obtaining high purity N-(2-hydroxyethyl)-2-pyrrolidone of little low-boiling component content, which serves as an intermediate material to be used for synthesis of N-vinyl-2-pyrrolidone.
Gamma-butyrolactone, which is one of starting materials of N-(2-hydroxyethyl)-2-pyrrolidone, exhibits inert behavior during synthesis of N-vinyl-2-pyrrolidone. When y-butyrolactone remains in N-(2-hydroxyethyl)-2-pyrrolidone, most of it remains also during synthesis of N-vinyl-2-pyrrolidone.
Boiling temperature of y-butyrolactone is close to that of synthesized N-vinyl-2-pyrrolidone, and their separation in later purification step is difficult. In consequence, it provides a cause for lowering purity and increasing purification costs of resulting N-vinyl-2-pyrrolidone.
On the other hand, when 2-aminoethanol, which also is one of starting materials for N-(2-hydroxyethyl)-2-pyrrolidone, remains in N-(2-hydroxyethyl)-2-pyrrolidone, various heterocyclic amines are side produced during synthesis of N-vinyl-2-pyrrolidone, to cause coloring or unpleasant odor of resulting N-vinyl-2-pyrrolidone.
We tried purification of N-(2-hydroxyethyl)-2-pyrrolidone by distillation, referring to those processes described in Official Gazettes cited earlier, but could not obtain through any of those processes N-(2-hydroxyethyl)-2-pyrrolidone of high purity fully satisfactory for use as the intermediate material for N-vinyl-2-pyrrolidone.
In particular, under the operation conditions and bottom liquid temperature conditions as given in JP 2001-2638 A and JP 2001-2640 A, N-(2-hydroxyethyl)-2-pyrrolidone which was useful as a starting material of N-vinyl-2-pyrrolidone was not obtained at all. Presumably the operation conditions as described in the cited Kokai Gazettes are not applicable to production of N-(2-hydroxyethyl)-2-pyrrolidone because its physical properties largely differ from those of 2-pyrrolidones or N-alkyl-pyrrolidones which are the object compounds of the inventions described in those Gazettes. For example, for attaining the bottom liquid temperature of 90-200xc2x0 C. following the descriptions in those Gazettes, inside pressure of the column needs to be kept no higher than 6.67 hPa (5.0 mmHg). In that occasion the column top temperature becomes about 10xc2x0 C., and costs for cooling the condenser notably increase to bring about economical disadvantage.
After extensive studies, we came to discover: (1) because N-(2-hydroxyethyl)-2-pyrrolidone has higher boiling point than N-alkyl-pyrrolidones, the object compounds named in the cited Gazettes, generally high temperature operations inside the distillation column are inevitable; (2) because the high-boiling compounds in the reaction liquid have different structures from those in the processes of the cited Gazettes and have high reactivity, a part of the high-boiling component contained in the reaction liquid composition at the production time of N-(2-hydroxyethyl)-2-pyrrolidone undergoes pyrolysis during the distillative purification to form xcex3-butyrolactone or 2-aminoethanol; (3) according to the processes as described in the cited Gazettes, accompanying of such xcex3-butyrolactone or 2-aminoethanol, which are formed of pyrolysis, to N-(2-hydroxyethyl)-2-pyrrolidone is unavoidable; and (4) those phenomena are the causes why N-(2-hydroxyethyl)-2-pyrrolidone of high purity fully satisfactory for use as the intermediate material for N-vinyl-2-pyrrolidone cannot be obtained.
From the above-acquired knowledge, it is clear that recovery of N-(2-hydroxyethyl)-2-pyrrolidone from the products of the process described in said JP1979-22973 B (=U.S. Pat. No. 3,867,405) by means of conventional distillation operations is liable to invite purity degradation of N-(2-hydroxyethyl)-2-pyrrolidone due to pyrolysis of the high-boiling component contained in the reaction liquid.
Accordingly, therefore, an object of the present invention is to provide a method for obtaining high purity N-(2-hydroxyethyl)-2-pyrrolidone which is fully satisfactory for use as an intermediate material for N-vinyl-2-pyrrolidone, from a reaction liquid resulting from reacting xcex3-butyrolactone with 2-aminoethanol and containing N-(2-hydroxyethyl)-2-pyrrolidone, low-boiling component and high-boiling component.
We have made concentrative research work with the view to accomplish the above object, to find that high purity N-(2-hydroxyethyl)-2-pyrrolidone can be efficiently and stably recovered by, in the occasion of the recovery through distillation of the liquid resulting from reacting xcex3-butyrolactone with 2-aminoethanol and containing N-(2-hydroxyethyl)-2-pyrrolidone, low-boiling component and high-boiling component, first removing the high-boiling component and then removing the low-boiling component and remaining high-boiling component, whereby inhibiting blending of impurities formed of pyrolysis of the high-boiling component into the object product.
Thus, according to the invention a method of purifying N-(2-hydroxyethyl)-2-pyrrolidone is provided, which process comprising purifying a liquid resulting from reacting xcex3-butyrolactone with 2-aminoethanol and containing N-(2-hydroxyethyl)-2-pyrrolidone, compounds having lower boiling points than that of N-(2-hydroxyethyl)-2-pyrrolidone, and compounds having higher boiling points than that of N-(2-hydroxyethyl)-2-pyrrolidone, by distillation using a distillation column, said process being characterized by obtaining a liquid containing said compounds having lower boiling points than that of N-(2-hydroxyethyl)-2-pyrrolidone and N-(2-hydroxyethyl)2-pyrrolidone as the liquid distillate from the column top, while obtaining a liquid containing the compounds having boiling points higher than that of N-(2-hydroxyethyl)-2-pyrrolidone as the bottom liquid.
According to the method of this invention, said distillation column (which is hereafter referred to as the first distillation column) is preferably operated under the conditions of 200-260xc2x0 C. in bottom liquid temperature and 6.67-66.7 hPa (5.0-50 mmHg) in column top pressure.
According to the method of the invention, the distillate liquid from the first distillation column is distilled using a second distillation column, whereby obtaining a liquid containing the compounds having boiling points lower than that of N-(2-hydroxyethyl)-2-pyrrolidone as the distillate liquid from the column top, and concurrently, purified N-(2-hydroxyethyl)-2-pyrrolidone as a side cut from a position of the distillation column higher than the bottom.
According to the method of the present invention, said distillate liquid distilled off from the first distillation column is distilled using the second distillation column, whereby obtaining a liquid containing the compounds having boiling points lower than that of N-(2-hydroxyethyl)-2-pyrrolidone as the distillate liquid from the column top and a liquid containing N-(2-hydroxyethyl)-2-pyrrolidone as the bottom liquid; and then said bottom liquid is distilled using a third distillation column to provide purified N-(2-hydroxyethyl)-2-pyrrolidone as the distillate liquid.
In short, according to the invention high purity N-(2-hydroxyethyl)-2-pyrrolidone can be obtained by separating the high-boiling component, which undergoes pyrolysis to form xcex3-butyrolactone or 2-aminoethanol, from N-(2-hydroxyethyl)-2-pyrrolidone in the distillation step using the first distillation column (formation of pyrolyzed products in this step has no detrimental effect), and then distilling the separated N-(2-hydroxyethyl)-2-pyrrolidone using the second distillation column or the second and third distillation columns.